1. Field of the Invention
The present invention relates to lead zirconate titanate (PZT) dielectric ceramic compositions, such as for use as piezoelectric ceramic transducer materials. PZT is the best and most widely used piezoelectric ceramic transducer material. The use of lead zirconate titanate (PZT) in cofirable multilayered electrode devices for electromechanical applications has been hindered by its reactivity with metal electrodes at its normal sintering temperatures.
Piezoelectric transducers are replacing electrodynamic or electromagnetic based devices for use as electromechanical transducers since they can be made smaller, more efficient (less heat produced) and have quicker response times. PZT is the best and hence the most widely used piezoelectric ceramic transducer material. However, PZT based actuators need relatively large electric fields (500 to 2000 V/mm) for maximum deflections and therefore they cannot be used easily in electronics where voltages range from 5 to 15 Volts. The solution to this problem is to fabricate PZT actuators in a multilayered configuration, similar to multilayered capacitors, by making thin tape-cast PZT sheets which are coated with electrode inks made of silver-palladium or platinum and then laminated and cofired. However, before this process can be used effectively the sintering temperature of PZT must be reduced from 1300 C. to about 1000-1100 C. or less. At the normal PZT sintering temperatures the electrode-ceramic interactions are highly detrimental. These interactions occur especially due to the high volatility of PbO in PZT. The gradual decrease in weight is due to the lead loss at temperatures greater than 950-1000 C. Therefore it is highly desirable to produce PZT compositions having substantially-reduced sintering temperatures, such as between about 900.degree.-950.degree. C., while retaining a high dielectric constant and minimizing dielectric losses.
2. Discussion of the State of the Art
It is well known to add sintering aids such as metal oxide and fluoride compounds to piezoelectric ceramic compositions in order to reduce the sintering temperature, lower manufacturing costs and enable the use of lower-melting, less expensive electrode metals. Reference is made to Cheng, S. Y., S. L. Fu, C. C. Wei and G. M. Ke, J. Mat. Sc., 21, pp. 571-576 (1986); Wittmer, D. E. and R. C. Buchanan, J. Amer. Ceramic Soc., 64, pp. 485-490 (1981), and Hennings, D., Ber. Dt. Keram. Ges. 55, pp. 359-360 (1978) Nr. 7, and U.S. Pat. Nos. 4,222,885 and 4,244,830. Sintering additives which are effective in certain ceramic compositions, such as barium titanates, may not be effective in PZT ceramic and/or may cause substantial degradation of the dielectric properties. Also prior known sintering aids for PZT compositions, such as vanadium pentoxide, lithium carbonate, sodium carbonate, boron oxide, bismuth oxide, etc., are not completely satisfactory. The prior known fluxes or processing techniques for lowering the sintering temperature of PZT ceramics are either too processing intensive for commercialization or tend to deteriorate the electrical properties considerably.
For large scale, low cost commercial applications the use of sintering aids is most appropriate. Typical conventional sintering aids are oxides of metals such as Li, Na, B, V, Bi and Pb, or their fluorides. A liquid phase sintering aid is typically chosen due to its melting temperature, which should be such that there is a liquid phase present during the second stage of sintering. This liquid phase makes it easier for particle rearrangement and for material transport enabling rapid and enhanced low temperature sintering. For high dielectric constant (K) compositions it is preferred that no second phase exists, but if present the equivalent circuit should be such that it is in parallel and not in series with the major high K phase.